Glyphosate does control Japanese knotweed, but only over several seasons and rarely in a single application. It is the herbicide at the heart of professional knotweed control, applied as a foliar spray or by stem injection so the plant carries it down into the rhizome, where the growth comes from. How it is used, and why patience is part of the method, is set out in our treatment methods, and a shop-bought bottle used once will not clear an established stand.
Does glyphosate kill Japanese knotweed?
Glyphosate kills Japanese knotweed by being drawn down from the leaves into the rhizome, where it disrupts the plant's ability to regrow, but this works only when the plant is in active growth and only across repeated seasonal treatments. A single application knocks back the visible canes without exhausting the rhizome, so the stand returns. Used correctly by a professional over 3 to 5 seasons, glyphosate brings the rhizome to a dormant, non-viable state, which is why it is the mainstay of knotweed control rather than a one-off fix.
How is glyphosate applied to knotweed?
Professionals apply glyphosate either as a foliar spray onto the leaves in the growing season, or by stem injection directly into each hollow cane, which concentrates the herbicide and suits sensitive sites near water or planting. The timing matters: late summer and autumn treatments are most effective because the plant is moving reserves down into the rhizome and carries the herbicide with them. A programme usually combines applications across the season and repeats them year on year until monitoring confirms the stand is spent.
Why does it take several seasons?
It takes several seasons because the rhizome stores large reserves and extends several metres, so no single dose reaches and exhausts all of it. Each treatment weakens the rhizome a little more, and the plant's own growth cycle limits how much herbicide it will take down in one year. This is why a credible treatment plan runs across 3 to 5 seasons with monitoring, and why any claim to kill knotweed in one visit with a spray should be treated with caution.
Can I use glyphosate myself?
You can buy glyphosate products, but clearing an established knotweed stand yourself is difficult, because domestic concentrations, timing and technique rarely reach the rhizome, and cut or moved material is controlled waste. Repeated mistimed spraying often just suppresses the canes while the rhizome survives, leaving a problem that still shows on a survey. For a stand that affects a sale or mortgage, a professional programme with an insurance-backed guarantee is what a lender will accept.
Common questions
- How long after glyphosate does knotweed die?
- The canes brown within weeks, but the plant is not dead, because the rhizome regrows the next spring. Only after several seasons of treatment and a monitoring period is a stand confirmed non-viable.
- Is stem injection better than spraying?
- Stem injection places glyphosate directly into each cane and suits sites near water, ponds or wanted planting where spray drift is a concern. Foliar spraying can be more practical on large open stands. The choice depends on the site.
- Does glyphosate harm other plants?
- Glyphosate is non-selective, so foliar spray can damage plants it touches, which is one reason stem injection is used near wanted planting. A professional applies it to limit off-target effect.